Conventional automotive starting, lighting, and ignition (SLI) batteries are 12 volt batteries consisting of six lead acid cells of approximately 2 volts each, connected in series. SLI batteries must respond to three different demands. Engine cranking requires high electric current for short intervals. Ignition at the spark plugs and coil requires lower current rates but for longer periods of time. Everincreasing numbers of vehicle auxiliaries such as lights, clocks, power windows and doors require low current rates, often while the generator is not working, leaving the battery as the only source of electrical energy.
These conflicting demands have evolved the traditional battery, designed mainly for cranking, into dual battery systems having main and reserve units. In one such system, the main battery is connected to the ignition and the reserve battery is connected to auxiliary systems such as lighting. See McDowall et al., U.S. Pat. No. 4,239,839 issued Dec. 16, 1980. The dual battery may have battery cells formed by integrally molded partition walls, as shown in Witehira U.S. Pat. No. 4,883,728 issued Nov. 28, 1989. This battery contains laminated plates of varying thicknesses to provide different discharge characteristics. In Arrance U.S. Pat. No. 3,607,403, issued Sep. 21, 1971, a self-charging battery unit has solid-fluid cell sections actuable to charge cell storage sections when the cell storage sections are discharged.
A variety of switched dual battery systems have been proposed. See, for example, Jordan et al., U.S. Pat. No. 3,475,221, issued Oct. 28, 1969 and Hughes U.S. Pat. No. 3,165,639, issued Jan. 12, 1965. Goebel U.S. Pat. No. 4,447,504, issued May 8, 1984, describes an electrochemical cell with two cell stacks capable of selective operation at one of several possible discharge rates. Noordenbos U.S. Pat. No. 4,770,954, issued Sep. 13, 1988 describes a two-energy source power supply wherein one energy source provides a high current but has a low energy density, and the other provides a low current but has a relatively high energy density. In Klebenow et al. U.S. Pat. No. 5,002,840, issued Mar. 26, 1991, a manual switch is used to bring the reserve battery into parallel with the main battery when needed.
Problems with manual switches include the inconvenience of having to manually engage a switch to access the reserve battery, operator error in using the switch if the vehicle fails to start for an unrelated reason, and operator failure to reverse the switch after the vehicle is started. Switch hardware renders the battery more complex and difficult to manufacture.
Automobiles are designed to accommodate a standard-sized, two-terminal battery. Non-standard battery sizes, remote circuitry, manual or electronic switches, and multiple terminals required by previous dual battery designs can be cost prohibitive. It is of practical and economic value to design a battery which fits the standard arrangements provided by automobile manufacturers, especially if it to be used as a replacement battery. The present invention can provide a two-terminal, standard sized dual battery having many of the same advantages as a switched dual battery system, together with other advantages known dual batteries lack.